The present invention generally relates to optical communication systems and more particularly to an apparatus for providing tailored multiple wavelength dispersion compensation.
Wavelength division multiplexing (WDM) is a technique for increasing the capacity of existing fiber optic networks by transmitting a plurality of channels over a single waveguide medium. WDM systems typically include a plurality of transmitters for transmitting modulated information signals on a designated one of a plurality of optical channels or wavelengths. The transmitters used in WDM systems typically include semiconductor lasers each transmitting on a designated one of a plurality of wavelengths. The selected wavelengths are usually within the 1.55 xcexcm range which corresponds to an absorption minimum associated with silica-based fibers. The channels are combined by a multiplexer at a first terminal and transmitted to a demultiplexer at a receiving terminal along a transmission fiber. One or more amplifiers may be positioned along the transmission fiber to optically amplify the transmitted signals. The demultiplexer separates the optical channels and supplies them to receiving circuitry which converts the optical signals into electrical signals for processing. Dense WDM (DWDM) systems are also employed with this same general construction, but have a greater number of optical channels, typically with smaller channel spacings.
The use of optical amplifiers in these types of systems solves the loss problem associated transmission over optical fiber, but does not solve the chromatic dispersion problem. Dispersion generally refers to the broadening of a transmitted pulse as it propagates down an optical fiber. Group velocity dispersion (GVD) is a parameter that expresses how much an optical pulse broadens when propagating inside an optical fiber and is expressed in units of ps/(km-nm). For xe2x80x9cstandardxe2x80x9d single mode optical fiber, the zero-dispersion wavelength xcexZD is≈1.31 xcexcm while a typical dispersion value for a wavelength in the 1.55 xcexcm range transmitted along the same xe2x80x9cstandardxe2x80x9d single mode fiber is 17 ps/(km-nm). As the pulses spread, they can overlap and interfere with each other, thereby impacting signal integrity. The effect becomes more pronounced at higher data rates. Pulses at different wavelengths typically suffer different amounts of dispersion. Therefore, in WDM systems where a plurality of channel wavelengths propagate along a single optical fiber, pulses at their respective wavelengths broaden at different rates. If multiple amplifiers are disposed along an optical fiber to accommodate long-haul signal transmission, the effects of dispersion on the transmitted signal accumulate over the path further impacting signal integrity.
Dispersion compensating fiber is a specialty optical fiber used to compensate for these dispersive effects encountered during signal transmission. Basically, the specialty fiber has a dispersion characteristic of opposite sign to the optical fiber used for transmission. Exemplary types of dispersion compensating fiber are commercially available from Lucent Technologies and/or Corning, Inc. (xe2x80x9cLSxe2x80x9d fiber). While dispersion compensating fiber is generally a broadband solution to first order dispersion (dispersion slope), it does not properly compensate for second order dispersion. That is, the optimum length of these specialty fibers varies with channel wavelength. Thus, in a WDM system where multiple wavelengths are transmitted, no one length of dispersion compensating fiber precisely accommodates all channel wavelengths.
To combat the effects of dispersion, some systems utilize dispersion shifted fiber. Dispersion shifted fiber can provide a transmission path with close to zero dispersion, however, it suffers from certain nonlinearities, such as four wave mixing, which affect signal integrity. Four wave mixing is a nonlinear effect that causes a plurality of waves propagating down a fiber at predetermined channel spacings to create a new wave at a particular frequency. This newly created wave causes crosstalk when it interferes with other channels within the signal channel plan. Therefore, with dispersion shifted fiber it is necessary to add back some dispersion to combat the effects of fiber nonlinearities. Accordingly, for transmission over standard optical fiber, dispersion compensating fiber is typically used at the receive end of a system to avoid costly installation of the specialty fiber within the transmission span.
Thus, there is a need to provide a simple and cost effective optical device tailored to provide a length or lengths of dispersion compensating fiber to properly compensate for second order dispersion in communication systems transmitting a plurality of optical channels.
The present invention meets these needs and avoids the above-referenced drawbacks by providing an optical device comprising a transmission path capable of carrying a plurality of optical channels, each at a respective wavelength. A wavelength branching element is coupled to the transmission path and is configured to separate at least one of the plurality of optical channels having a particular wavelength from the multiplexed signal. A segment of dispersion compensating optical fiber is coupled to the wavelength branching element. The segment of optical fiber has a length corresponding to the particular wavelength associated with the at least one of the plurality of optical channels.
The foregoing, and other features and advantages of the present invention, will be apparent from the following description, the accompanying drawings and the appended claims.